Spin relaxation and decoherence of holes in quantum dots

We investigate heavy-hole spin relaxation and decoherence in quantum dots in perpendicular magnetic fields. We show that at low temperatures the spin decoherence time is 2 times longer than the spin relaxation time. We find that the spin relaxation time for heavy holes can be comparable to or even longer than that for electrons in strongly two-dimensional quantum dots. We discuss the difference in the magnetic-field dependence of the spin relaxation rate due to Rashba or Dresselhaus spin-orbit coupling for systems with positive (i.e., GaAs quantum dots) or negative (i.e., InAs quantum dots) g factor.     

Electrical control of spin relaxation in a quantum dot

We demonstrate electrical control of the spin relaxation time T1 between Zeeman-split spin states of a single electron in a lateral quantum dot. We find that relaxation is mediated by the spin-orbit interaction, and by manipulating the orbital states of the dot using gate voltages we vary the relaxation rate W identical withT1(-1) by over an order of magnitude. The dependence of W on orbital confinement agrees with theoretical predictions, and from these data we extract the spin-orbit length. We also measure the dependence of W on the magnetic field and demonstrate that spin-orbit mediated coupling to phonons is the dominant relaxation mechanism down to 1 T, where T1 exceeds 1 s.     

Relationship between structural and stress relaxation in a block-copolymer melt

The relationship between structural relaxation on molecular length scales and macroscopic stress relaxation was explored in a disordered block-copolymer melt. Experiments show that the structural relaxation time, measured by x-ray photon correlation spectroscopy is larger than the terminal stress relaxation time, measured by rheology, by factors as large as 100. We demonstrate that the structural relaxation data are dominated by the diffusion of intact micelles while the stress relaxation data are dominated by contributions due to disordered concentration fluctuations.     

Dielectric relaxation in nanopillar NiFe–silicon structures in high magnetic fields

We explore the dielectric relaxation properties of NiFe nanowires in a nanoporous silicon template. Dielectric data of the NiFe–silicon structure show a strong relaxation resonance near 30 K. This system shows Arrhenius type of behavior in the temperature dependence of dissipation peaks vs. frequency. We report magnetic field dependence of dipolar relaxation rate and the appearance of structure in the dielectric spectrum related to multiple relaxation rates. A magnetic field affects both the exponential prefactor in the Arrhenius formula and the activation energy. From this field dependence we derive a simple exponential field dependence for the prefactor and linear field approximation for the activation energy which describes the data. We find a significant angular dependence of the dielectric relaxation spectrum for regular silicon and nanostructured silicon vs. magnetic field direction, and describe a simple sum rule that describes this dependence. We find that although similar behavior is observed in both template and nanostructured materials, the NiFe–silicon shows a more complex, magnetic field dependent relaxation spectrum.     

Time resolved scattering relaxation mechanisms of microcavity polaritons

We study the polariton relaxation dynamics for different scattering mechanisms as: Phonon and electron scattering procesess. The relaxation polariton is obtained at very short times by solving the Boltzman equation. Instead of the well-known relaxation process by phonons, we show that the bottleneck effect relaxes to the ground state more efficiently at low pump power intensity when the electron relaxation process is included. In this way, we clearly demonstrate that different relaxation times exist, for which any of these two mechanism is more efficient to relax the polariton population to the ground state.     

Slow algebraic relaxation in quartic potentials and related results

We present a detailed report [see S. Sen et al., Phys. Rev. Lett. 77, 4855 (1996)] of our numerical and analytical studies on the relaxation of a classical particle in the potentials V(x)= +/-x(2)/2+x(4)/4. Both of the approaches confirm that at all temperatures, the relaxation functions (e.g., velocity relaxation function and position relaxation function) decay asymptotically in time t as sin(omega(0)t)/t. Numerically calculated power spectra of the relaxation functions show a gradual transition with increasing temperature from a single sharp peak located at the harmonic frequency omega(0) to a broad continuous band. The 1/t relaxation is also found when V(x) is a polynomial in powers of x(2) with a nonvanishing coefficient accompanying the x(4) term in V(x). Numerical calculations show that in the cases in which the leading term in V(x) behaves as x(2n) with integer n, the asymptotic relaxation exhibits 1/t(phi) decay where phi =1/(n-1). We briefly discuss the analytical approaches to relaxation studies in these strongly anharmonic systems using direct solution of the equation of motion and using the continued fraction formalism approach for relaxation studies. We show that the study of the dynamics of strongly anharmonic oscillators poses unique difficulties when studied via the continued fraction or any other time-series construction based approaches. We close with comments on the physical processes in which the insights presented in this work may be applicable.     

Two-photon coincidence studies of highly-charged ion relaxation in solids

We studied the relaxation of hollow atoms by two-photon transitions during the stopping of slow (E=8.5q keV) highly charged Pb^q+ (q=53–58) ions in a thin Ta foil. The X-rays emitted during the relaxation process were detected in coincidence with two Si(Li) detectors. Particular attention was paid to the role of the internal dielectronic excitation (IDE) in the relaxation dynamics. We found a clear contribution to relaxation by the IDE process also for projectiles with initial M-shell vacancies and IDE speeds up relaxation of electrons from high Rydberg states by directly filling vacancies in the N-shell that is accompanied with the excitation of M-shell electrons. We discuss the relaxation mechanism, and present results for the fluorescence yield and the IDE probability of highly charged Pb ions in a solid.     

Limits to the polarization for spin-exchange optical pumping of 3He

Based on measurements of the temperature dependence of 3He relaxation in a wide range of spin-exchange optical pumping cells, we report evidence for a previously unrecognized surface relaxation process. The relaxation rate was found to be linearly proportional to the alkali-metal density with a slope that exceeds the spin-exchange rate, which limits the polarization for current applications, including neutron spin filters, polarized targets, and polarized gas magnetic resonance imaging. We find that the magnitude of this excess relaxation can vary widely between cells, and that the variation is larger for cells of higher surface to volume ratio. We have observed 3He polarization as high as 81%, but further improvements require understanding the origin of this relaxation.     

129Xe-Xe molecular spin relaxation

We identify the formation of bound 129Xe-Xe molecules as the primary fundamental spin-relaxation process at densities below 14 amagat. Low pressure Xe relaxation rate measurements as a function of gas composition show that Xe-Xe molecular relaxation contributes 1/T1 = 1/4.1 h to the total observed relaxation rate. The measured rate is consistent with theoretical estimates deduced from previously measured NMR chemical shifts. At atmospheric pressure the molecular relaxation is more than an order of magnitude stronger than binary relaxation. Confusion of molecular and wall relaxation mechanisms has historically caused wall relaxation rates to be overestimated.     

Mesoscopic charge relaxation

We consider charge relaxation in the mesoscopic equivalent of an RC circuit. For a single-channel, spin-polarized contact, self-consistent scattering theory predicts a universal charge relaxation resistance equal to half a resistance quantum independent of the transmission properties of the contact. This prediction is in good agreement with recent experimental results. We use a tunneling Hamiltonian formalism and show in Hartree-Fock approximation that at zero temperature the charge relaxation resistance is universal even in the presence of Coulomb blockade effects. We explore departures from universality as a function of temperature and magnetic field.     

Ultrafast carrier dynamics of resonantly excited InAs/GaAs self-assembled quantum dots

We study theoretically the time development of electronic relaxation in quantum dots. We consider the process of relaxation of the state with an electron prepared at the beginning of relaxation in the electronic ground state. We obtain a fast (in picoseconds) increase of electronic population in the excited state. Also, we consider the process of relaxation of an electron from an excited state in the dot. Here we obtain an incomplete depopulation of the electron from the excited state. We compare these results to experiments in which a fast decrease of luminescence is reported during the first period of relaxation after resonant excitation of the ground state. We estimate numerically the role of electron–LO–phonon (Fröhlich's coupling) mechanism in these processes. We show that this effect may be attributed to the influence of multiple scattering of quantum dot electrons on LO phonons. A single-electron two-energy-level quantum dot model is used to demonstrate this effect in an isolated semiconductor quantum dot.     

Complex local dynamics in DNA on the picosecond and nanosecond time scales

Time-resolved Stokes shift measurements of the local structural relaxation of three DNA oligonucleotides are presented. Logarithmic relaxation is seen for over three decades in time (40 ps-40 ns), indicating a complex relaxation among a large number of conformational substates. The observed relaxation is the same in all the sequences. Sequence dependence of the localized dynamics of DNA does not appear within this time range. We infer that 30%-50% of the relaxation is faster than 40 ps, has a nonlogarithmic decay and has a sequence dependent amplitude.     

Anisotropic local stress and particle hopping in a deeply supercooled liquid

The origin of the microscopic motions that lead to stress relaxation in deeply supercooled liquid remains unclear. We show that in such a liquid the stress relaxation is locally anisotropic which can serve as the driving force for the hopping of the system on its free energy surface. However, not all hoppings are equally effective in relaxing the local stress, suggesting that diffusion can decouple from viscosity even at the local level. On the other hand, orientational relaxation is found to be always coupled to stress relaxation.     

Coulomb "Blockade" of nuclear spin relaxation in quantum dots

We study the mechanism of nuclear spin relaxation in quantum dots due to the electron exchange with the 2D gas. We show that the nuclear spin relaxation rate 1/T(1) is dramatically affected by the Coulomb blockade (CB) and can be controlled by gate voltage. In the case of strong spin-orbit (SO) coupling the relaxation rate is maximal in the CB valleys, whereas for the weak SO coupling the maximum of 1/T(1) is near the CB peaks.     

Relaxation at critical points: Deterministic and stochastic theory

A generalized critical point can be characterized by non-linear dynamics. We formulate the deterministic and stochastic theory of relaxation at such a point. Canonical problems are used to motivate the general solutions. In the deterministic theory, we show that at the critical point certain modes have polynomial (rather than exponential) growth or decay. The stochastic relaxation rates can be calculated in terms of various incomplete special functions. Three examples are considered. First, a substrate inhibited reaction (marginal type dynamical system) is treated. Second, the relaxation of a mean field ferromagnet is considered. We obtain a result that generalizes the work of Griffiths et al. Third, we study the relaxation of a critical harmonic oscillator.     

Influence of degeneracy on momentum relaxation times in semiconductors

We have studied the effect of degeneracy on momentum relaxation times under ohmic as well as non-ohmic conditions. We find that a proper momentum relaxation time, within the framework of the Boltzmann transport equation, can no longer be defined for isotropic but inelastic scattering when the carriers are hot whereas under ohmic conditions it can be defined and is appreciably altered by degeneracy. For elastic scattering the momentum relaxation time is found to be unaffected by degeneracy for both ohmic and non-ohmic regimes.     

The interpretation of multi-exponential water proton transverse relaxation in the human and porcine eye lens

We report results of 1H NMR transverse relaxation experiments on human and porcine eye lenses. Several authors have reported that transverse relaxation is not mono-exponential when observed by the Carr-Purcell-Meiboom-Gill (CPMG) sequence and have interpreted the results by postulating the presence of "pools" of water molecules in different binding environments that do not exchange rapidly on the NMR timescale. We have compared CPMG data for intact lenses with results for lens homogenates and have combined a CPMG spectroscopic pulse train with NMR micro-imaging to study the nature of the transverse relaxation process in human and porcine lenses. Fast exchange of water protons with the lens proteins (crystallins) leads to an enhanced transverse relaxation rate that varies linearly with protein concentration. At the resolution of NMR micro-imaging the transverse relaxation process is mono-exponential. The results show that the multi-exponential CPMG data observed spectroscopically for whole lenses reflect spatial variations in crystallin content through the lens rather than the presence of distinct "bound" and "free" water pools.     

Nonlinear viscoelastic dynamics of nanoconfined wetting liquids

The viscoelastic dynamics of nanoconfined wetting liquids is studied by means of atomic force microscopy. We observe a nonlinear viscoelastic behavior remarkably similar to that widely observed in metastable complex fluids. We show that the origin of the measured nonlinear viscoelasticity in nanoconfined water and silicon oil is a strain rate dependent relaxation time and slow dynamics. By measuring the viscoelastic modulus at different frequencies and strains, we find that the intrinsic relaxation time of nanoconfined water is in the range 0.1-0.0001 s, orders of magnitude longer than that of bulk water, and comparable to the dielectric relaxation time measured in supercooled water at 170-210 K.     

Dielectric relaxation spectrum of water studied by the site—site generalized Langevin/modified mode-coupling theory

The dielectric relaxation spectrum of water is calculated from the site-site generalized Langevin/modified mode-coupling theory. The main part of the relaxation follows the Debye-type function, and a small deviation from the Debye relaxation is found on the high-frequency side. This tendency is consistent with recent experiments, although the absolute relaxation time does not agree with the experimental value quantitatively. The time development of the longitudinal polarization function resembles the dielectric part of the memory function, and we consider that this is because the dielectric friction dominates the collective reorientation of the dipole moment of water. We performed calculations with different dielectric constants using the reference interaction-site model integral equation, and found that the large gap between the time scales of the dielectric relaxation and the longitudinal polarization relaxation causes the Debye-type dielectric relaxation in our theory when the dielectric friction is dominant in the friction on the collective reorientation of the dipole moment. Namely, the longitudinal polarization relaxation is fast enough to be considered as a white noise to the dielectric relaxation process, so that the relaxation becomes a Markov process. The large gap between the two relaxation times originates from a large local field correction owing to the large dielectric constant of water. It is also suggested that the deviation from the Debye relaxation at the high-frequency side is the manifestation of the slow memory caused by the long-time part of the longitudinal polarization relaxation in the low-wavenumber region.     

Ageing of natural rubber under stress

We report a dynamical-mechanical study of stress relaxation at small deformation in a natural (polyisoprene) rubber well above its glass transition temperature . We find that an almost complete relaxation of stress takes place over very long relaxation periods, even though the elastic network is retained. The relaxation rate and the long-time equilibrium modulus are sensitive functions of temperature which do not follow time-temperature superposition. Many characteristic features of non-ergodic ageing response are apparent at both short and very long times. We interpret the observed behaviour in terms of the nature of rubber cross-links, capable of isomerisation under stress, and relate the results to recent models of slow glassy rheology. Received 22 November 1999 and Received in final form 18 January 2000